net: e1000: Add support for the Intel 82546GB controller

This chip is equipped for example on the esd PMC-ETH2-GB board. So let's
add it to the list of supported chips to the e1000 driver.

Signed-off-by: Reinhard Arlt <reinhard.arlt@esd.eu>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>